This invention relates to a low pressure arc discharge lamp comprising a discharge tube with at least one sealed end. The sealed end contains a filament supported by lead-in wires. The lamp is provided with an end-of-life structure.
Low pressure discharge lamps are well known in the art. These lamps exhibit a characteristic failure when the electrode emissive material on at least one of the filaments has been depleted. This form of failure is termed hereinafter as the end-of-life of the lamp. When the emitter material disappears from the filament, the voltage across the lamp increases, and the arc current in the discharge tube dissipates substantially increased power on the electrodes. As a result, unwanted heating effects occur.
Several solutions were suggested to limit this unwanted performance of the lamp. U.S. Pat. No. 5,210,461 discloses a lamp with an end-of-life structure, comprising a filament which is in physical contact with the wall of the discharge tube. When the filament is heated due to the end-of-life effect, the direct heating of the wall causes it to crack. The arc is extinguished by the outer atmosphere entering the discharge tube. This solution has the drawback that the filament scratches off the phosphor coating within the discharge tube which negatively affects the visual appearance of the lamp. Also, the localized heating of the wall may cause excessive fracturing of the discharge tube. Therefore, a further external cover is needed on the end portion of the discharge tube.
U.S. Pat. No. 5,446,340 discloses a discharge lamp which is provided with a structural weakening of the discharge tube. The structural weakening is formed on the pinched ends of the tube. The purpose of the structural weakening is to cause a cracking of the tube when the temperature of the pinched ends surpasses the normal operating temperature. This solution has the drawback that the cracking of the tube occurs only after a relatively long time. During this time the heating of the pinched ends causes the melting of the plastic housing of the lamp. The melting of the plastic may develop irritating smell, and may turn on smoke detector devices, causing false alarm.
Therefore, there is a need for a low pressure discharge lamp which exhibits controlled end-of-life failure in a relatively short time after the depletion of the electron emitting material without causing the melting of the plastic housing, and which lamp may be manufactured economically.
In an embodiment of the present invention, there is provided a low pressure arc discharge lamp comprising a discharge tube with at least one sealed end. The sealed end contains a filament for forming a discharge arc. The filament is supported by lead-in wires. The discharge lamp further comprises a connecting element which transversally connects the lead-in wires within the discharge tube. This connecting element is made of an insulating material. The transversal dimension of the connecting element is selected so as not to allow the discharge arc beyond the connecting element upon end-of-life of the discharge lamp.
The term xe2x80x9cend-of-lifexe2x80x9d, as mentioned above, is defined as the failure of the lamp due to the depletion or disappearance of the electron emitter material from the filament. This failure effect is well known in the art, and it is also described in U.S. Pat. No. 5,210,461.
In a further refinement of the lamp, it is foreseen that the discharge tube comprises a structural weakening in the vicinity of the connecting element. This structural weakening may take different forms, the most expedient being a reduction of the discharge tube wall thickness.
The structural weakening contributes to the guaranteed cracking of the discharge tube when the discharge arc strikes the tube wall. Alternatively, the arc causes the melting of the connecting element, and the tube wall cracks upon physical contact with the hot melted material.
It is suggested to use a lamp configuration where the structural weakening is external to the lamp housing. In this manner, the melting of the lamp housing itself is largely prevented, and the above mentioned negative effects are avoided.